1. Field of the Invention
The invention relates generally to seals used with roller cone drill bits. More specifically, the invention relates to rotary seals that include more than one dynamic sealing surface for maintaining lubrication in roller cone drill bits.
2. Background Art
Drill bits are commonly used in, for example, the oil and gas exploration industry for drilling wells in earth formations. One type of drill bit commonly used in the industry is the roller cone drill bit. Roller cone drill bits generally comprise a bit body connected to a drill string or bottom hole assembly (BHA). Roller cone drill bits typically include a plurality of roller cones rotatably attached to the bit body. The roller cones are generally mounted on steel journals integral with the bit body at its lower end. The roller cones further comprise a plurality of cutting elements disposed on each of the plurality of roller cones. The cutting elements may comprise, for example, inserts (formed from, for example, polycrystalline diamond, boron nitride, and the like) and/or milled steel teeth that are coated with appropriate hardfacing materials.
When drilling an earth formation, the roller cone drill bit is rotated in a wellbore, and each roller cone contacts the bottom of the wellbore being drilled and subsequently rotates with respect to the drill bit body. Drilling generally continues until, for example, a bit change is required because of a change in formation type is encountered in the wellbore or because the drill bit is worn and/or damaged. High temperatures, high pressures, tough, abrasive formations, and other factors all contribute to drill bit wear and failure.
When a drill bit wears out or fails as the wellbore is being drilled, it is necessary to remove the BHA from the well so that the drill bit may be replaced. The amount of time required to make a bit replacement trip produces downtime in drilling operations. The amount of downtime may be significant, for example, when tripping in and out of relatively deep wells. Downtime can add to the cost of completing a well and is a particular problem in offshore operations where costs are significantly higher. It is therefore desirable to maximize the service life of a drill bit in order to avoid rig downtime.
One reason for the failure of a roller cone drill bit is the wear that occurs on the journal bearings that support the roller cones. The journal bearings may be friction-type or roller-type bearings, and the journal bearings are subjected to high loads, high pressures, high temperatures, and exposure to abrasive particles originating from the formation being drilled. The journal bearings are typically lubricated with grease adapted to withstand tough drilling environments, and such lubricants are an important element in the life of a drill bit.
Lubricants are retained by a journal bearing seal, which is typically an O-ring type seal. The seal is typically located in a seal groove formed on an interior surface of a roller cone. The seal generally includes a static seal surface adapted to form a static seal with the interior surface of the roller cone and a dynamic seal surface adapted to form a dynamic seal with the journal upon which the roller cone is rotatably mounted. The seal must endure a range of temperature and pressure conditions during the operation of the drill bit to prevent lubricants from escaping and/or contaminants from entering the journal bearing. Elastomer seals known in the art are conventionally formed from a single type of rubber or elastomeric material, and are generally formed having identically configured dynamic and static seal surfaces with a generally regular cross section.
The rubber or elastomeric material selected to form the seal for the journal bearings has particular hardness, modulus of elasticity, wear resistance, temperature stability, and coefficient of friction, among other properties. Additionally, the particular geometric configuration of the seal surfaces produces a selected amount of seal deflection that defines the degree of contact pressure or xe2x80x9csqueezexe2x80x9d applied by the dynamic and static seal surfaces against respective journal bearing and roller cone surfaces.
The wear, temperature, and contact pressures encountered at the dynamic seal surface are different than those encountered at the static seal surface. Therefore, the type of seal material and seal geometry that is ultimately selected to form both seal surfaces represents a compromise between satisfying the operating conditions that occur at the different dynamic and static seal surfaces. Conventional seals formed from a single-type of material, having symmetric axial cross-sectional geometries, may have reduced wear resistance and temperature stability at the dynamic seal surface where wear and temperature conditions are generally more severe than at the static seal surface. Therefore, the service life of drill bits that contain such seals may be limited by the service life of the journal bearing seal.
There have been several attempts to produce tough, long-lasting journal bearing seals that satisfy the requirements of both dynamic and static sealing of roller cone journal bearings. For example, U.S. Pat. No. 3,765,495 discloses a drill bit seal that has a greater radial cross section than axial cross section by a ratio of at least 1.5:1. The seal, which may be referred to as a xe2x80x9chigh aspect ratio seal,xe2x80x9d has a symmetrical, generally rectangular axial cross section and is made from a single type of elastomer. The seal has identically configured dynamic and static surfaces, and is formed from a single type of elastomeric material, reflecting a compromise between meeting the different operating conditions at each seal surface.
U.S. Pat. No. 5,362,073 discloses a composite drill bit seal formed from two or more different materials selected to provide a desired degree of wear resistance at the dynamic seal surface, and to provide a desired degree of seal contact at the static seal surface. The seal has a dynamic seal surface on its internal diameter formed from a single type of elastomeric material, and has inner and outer seal surfaces that are each formed from a different material than the other. Further, the dynamic seal surface has a radius of curvature less than that of each static seal surface.
U.S. Pat. Nos. 6,170,830 and 6,179,276 disclose drill bit seals that have asymmetric cross sections and that are formed from different elastomeric materials. The seals are circular in shape and are adapted to form a dynamic seal with a bearing journal on an inner face of the seal and a static seal with a surface of a roller cone on an outer face of the seal.
Prior art seals are generally adapted to form dynamic seals on inner surfaces and static seals on outer surfaces thereof. For example, the OD seal surface of prior art seal designs are arranged to form a static seal with an internal surface of a seal gland (where the seal gland is formed on an internal surface of a roller cone). During operation, if, for example, an increase in the operating temperature causes a decrease in desirable properties of the seal elastomer, the ID seal surface may become static by sticking, and the OD seal surface then becomes dynamic. When rotation occurs at the OD seal surface, which is usually formed from a relatively soft elastomer and has a relatively poor wear resistance, the OD seal surface experiences severe wear and may fail after a short time.
It is desirable to produce a seal that is capable of forming dynamic seals on both inner and outer surfaces to compensate, for example, for xe2x80x9cstick-slipxe2x80x9d conditions where rotation relative to the drill bit body and/or the roller cone occurs adjacent the inner surface of the seal, the outer surface of the seal, or adjacent to both inner and outer surfaces of the seal simultaneously.
One aspect of the invention is a roller cone drill bit including a seal adapted to seal between the bit body and a roller cone rotatably mounted on the bit body. The seal comprises a seal body formed from a material adapted to energize the seal after compression thereof, a first dynamic seal surface formed on the seal body and adapted to seal against the bit body, and a second dynamic seal surface formed the seal body and adapted to seal against the roller cone. The first and second dynamic seal surfaces are formed from a material adapted to withstand relative motion between the first dynamic seal surface and the bit body and between the second dynamic seal surface and the roller cone.
Another aspect of the invention is a drill bit including a seal adapted to seal between the bit body and a roller cone rotatably mounted on the bit. The seal comprises a seal body formed from a material adapted to energize the seal after lateral compression thereof, and the seal body also comprises a recessed upper axial surface and a recessed lower axial surface. The recessed upper and lower axial surfaces of the seal body are adapted to axially expand in a selected manner after lateral compression of the seal. The seal comprises a first dynamic seal surface formed on an inner diameter of the seal body and adapted to seal against the bit body and a second dynamic seal surface formed on an external diameter of the seal body and adapted to seal against the roller cone. The first and second dynamic seal surfaces are formed from a material adapted to withstand relative motion between the first dynamic seal surface and the bit body and between the second dynamic seal surface and the roller cone.
Another aspect of the invention is a drill bit comprising a bit body, at least one roller cone rotatably attached to the bit body, and a seal disposed between a first sealing surface located on the bit body and a second sealing surface located on the at least one roller cone. The seal further comprises a seal body formed from a material adapted to energize the seal after lateral compression thereof, a first dynamic seal surface disposed on an internal diameter of the seal body and adapted to seal against the first sealing surface, and a second dynamic seal surface disposed on an external diameter of the seal body and adapted to seal against the second sealing surface. The first and second dynamic seal surfaces formed from a material adapted to withstand relative motion between the first dynamic seal surface and the first sealing surface and between the second dynamic seal surface and the second sealing surface.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.